Various hydro-mechanical transmission arrangements known in the art have been proposed for use in vehicles and the like as the primary variable speed transmission. Such arrangements may also be employed as steering mechanism for track-type tractors. For example, a hydro-mechanical variable ratio transmission is disclosed in U.S. Pat. No. 3,905,251 issued Sep. 16, 1975 to Clarence Kirk Greene. This transmission utilizes either a planetary gear mechanism or a bevel gear type differential mechanism to combine the mechanical and hydraulic inputs. While an in-line motor is disclosed in one embodiment of this patent, such motor is not revolvable, thus necessitating the use of separate gearing, adding to the complexity and cost and preventing compactness of the unit.
An in-line hydro-mechanical transmission is disclosed in U.S. Pat. No. 3,190,074, issued Jan. 22, 1965 to S. S. Johns. This transmission has a revolvable mechanism containing a charging pump, a main pump and a fluid motor. As the pump is integral with the transmission, rather than being remote, the system is extremely complex and requires a mechanical connection to the pump rotor for changing the speed or power ratio of the transmission. Incorporation of the main and charging pumps into the transmission also increases the size of the entire unit, making its use impractical where available space is at a minimum. Also, the vane type of pump and motor construction disclosed is not compatible with radial or axial piston motor designs.
In U.S. Pat. No. 3,643,434, issued Feb. 22, 1972 to Dieter Widmaier, a dual hydraulic unit is disclosed which can be used as a double pump or double motor, or as a hydrostatic transmission. Here, as in the preceding prior art device, the dual pump/motor arrangement makes the unit large and complex. Further, such dual device does not provide a gearless transmission with separate mechanical and hydraulic inputs and a combined output. Also, the dual axial piston pump/motor design is not fully compatible to a radial piston motor design.